Phosphotriesterase (PTE) is an enzyme from the soil bacteria Pseudomonas diminata that catalyzes the hydrolysis of a number of synthetic phosphotriesterase compounds used as insecticides and chemical warfare agents. T.S. Scanlan and R.C. Reid at UCSF have found that the translated sequence of an open reading frame (ORF) in E.coli is significantly homologous to the amino acid sequence of PTE, with 28% identity and 66% similarity over the entire sequence of the E.coli ORF. The three- dimensional structure of PTE has been solved, which provides a starting point to model Phosphotriesterase Homology Protein (PHP). The poject involves extensive molecular modeling of PHP and its probable active site using the facilities of the Computer Graphics Laboratory. These models may give us clues of PHP's natural function, which remains to be determined.